1. Field of the Invention
The present invention relates to a millimeter wave image processor and a millimeter wave image processing method, in which an electromagnetic wave of a millimeter wave emitted or reflected by a moving or still object is received, whereby image information of the object as a target is acquired.
2. Description of Related Art
A human body or the like may emit a millimeter wave together with an infrared ray. Further, every article emits and reflects an electromagnetic wave of a millimeter wave band at an intensity determined depending on the temperature and the radiative coefficient. The transparency of a millimeter wave with respect to moisture and substance is higher than the transparency of an infrared ray with respect to moisture and substance. By receiving such a millimeter wave so as to perform image processing, it is possible to monitor a target such as a distant landscape where is interrupted by fog and a near human body wearing a thick coat, and the like. This includes a case where an airplane makes a landing in the fog or a case of monitoring passengers passing through a security gate at an airport.
As a method of monitoring such a target, a method of monitoring by obtaining a visual image of high accuracy has been proposed as shown in Japanese Patent Application Laid-Open No. 6-331725 (Patent Document 1). The art shown in Patent Document 1 was developed considering the fact that image scanning within a field of view took time as accuracy became higher, whereby imaging of a moving object became difficult. To cope with it, in Patent Document 1, a plurality of receiving elements is provided to speed up scanning.
In Patent Document 1, in order to make an image highly accurate, the antenna aperture size must be large. However, in a large antenna of a millimeter wave band, extremely high machining accuracy is needed, and the manufacturing is difficult, and a thermal distortion error caused by temperature change in the used environment cannot be disregarded. Further, such an antenna is designed to obtain a narrow beam width and high gain in a far field, so if an object is in a near field, the resolution will be lowered after all. Moreover, in order to scan within a certain view angle with a large antenna of high resolution, the integral time per resolution cannot be obtained sufficiently as the beam width becomes acuter, so that the sensitivity is lowered. Therefore, it is difficult to obtain an image with high accuracy.
In view of the above, a radar system in which a phase error caused by antenna distortion and temperature characteristics of a cable is compensated has been developed, as disclosed in Japanese Patent Application Laid-Open No. 2003-177175 (Patent Document 2). In the art disclosed in Patent Document 2, a millimeter wave from an object is received by a two-dimensional plane antenna, and the received signal is converted by an A/D converter to digital data, and the digital signal is processed by a signal processor to thereby obtain image data. In Patent Document 2, prior to measurement, a calibration signal is output from the calibration signal source, and phase correction data is obtained by a compensator of the signal processing unit to thereby perform correction of measured data.
However, in the processing method disclosed in Patent Document 2, when monitoring a target positioned in a far field, an emitted signal from a target is viewed as a plane wave so as to be able to generate an image of sufficiently high resolution. However, when monitoring a target located in a near field, a signal from a target made incident on an antenna is close, so it is not considered as a plane wave. This causes a problem of unfocused image.
Further, as the frequency band width for the processing is wider, higher sensitivity can be realized. However, the phase delay difference among all antenna systems in the wide receiving band width must be small. This makes the manufacturing of the system difficult and expensive.